I have a Raspbery Pi Zero W that was working, but today seems to be stuck in a boot loop.
I'm running Raspian Lite on it. The boot messages look normal as far as I can tell. The last message I see says "Check for raspberry Pi EEPROM update", and then it restarts.
I tried using DietPi, but the board shows have similar symptoms. It gets through interactive setup, but when I rebooted (after trying to turn on WiFi) DietPi also restarts before it gets to a login prompt. It generates far fewer messages; the last I see is about setting the CPU governor, but I don't think that's too meaningful because it doesn't log much.
One clue might be that I was fussing with the watchdog timer this morning. (I followed this tutorial, including the fork bomb test. The fork bomb worked and my Pi restarted. At the next reboot, my reboot loop symptoms started.) In fact, initially assumed that the watchdog was firing during the boot. But since I've done a clean install with a new image, wouldn't all the watchdog settings be gone? Are any persisted in hardware, and not reset with a fresh install?
Why would it be that I can get to the text-mode setup in DietPi, but Raspian Lite's text-mode setup can't boot?
Any ideas on how I can diagnose what's wrong? Some way to reset the device? Am I just sunk?
Per a comment below, I tried with NOOBS. It boots, then resizes the volume. It rebots, then shows the rainbow screen. It quickly shows the raspberry screen with "Hold SHIFT", then reboots to the rainbow screen again. I've tried holding SHIFT--the pattern continues.